ABSTRACT RNA interference (RNAi) has the potential to revolutionize treatment of dominant genetic disorders. Small inhibitory RNAs (siRNAs) are highly potent and selective, demonstrating remarkable single-nucleotide specificity. Clinical trials using siRNAs are currently underway for a number of indications including age- related macular degeneration and respiratory syncitial virus infection. No siRNA treatment of skin disorders has been introduced to date into the clinic. In this proposal we aim to enable and initiate a clinical trial of pachyonychia congenita (PC), an ultra-rare skin disorder resulting from mutations, including single nucleotide changes, in genes encoding keratins 6, 16, and 17. The major complaint of PC patients is the debilitating painful callusing and blistering which occurs on or near the pressure points of the feet. It is these defined regions on the soles of the feet that are targeted for local siRNA treatment. In Phase 1 of this proposal, we extend previous studies investigating the potency, duration and selectivity of the proposed lead siRNA therapeutic as well as perform an FDA IND-enabling toxicology study in rabbits. A 28-day tox study was recently completed in mice, with expected localized toxicity occurring at the higher doses. The proposed rabbit study fulfills the second species toxicology requirement of the FDA. In Phase 2 of this proposal, all eligible and willing U.S. patients containing the K6a N171K mutation will be treated in a Phase 1b clinical trial. We expect this trial to be the "first-in-man" for siRNA treatment in skin. Although pachyonychia congenita is a rare disease, the nature of the disorder makes it an ideal prototype skin disorder (defined mutations with expression in limited, defined areas) for an initial siRNA clinical trial and the lessons learned should be readily generalized to other skin disorders. PUBLIC HEALTH RELEVANCE Despite the discovery of the underlying mutations responsible for many inherited skin diseases, few effective treatments have emerged. The discovery that siRNAs can be developed to target specific disease- related mutations portends the advent of a new treatment paradigm. We have identified a potent and selective siRNA that targets a single nucleotide mutation in the keratin 6a gene that is responsible for the rare skin disorder pachyonychia congenita and blocks its expression. This siRNA (known to the FDA as TD101) has been granted orphan drug status and is in the final stage of IND-enabling testing (including toxicology safety testing proposed in Phase 1) prior to initiating a Phase 1b clinical trial (Phase 2 of this proposal). The lessons learned in applying siRNA technology for treatment of PC should be readily generalizable to a host of other dominant skin disorders and likely to other disorders resulting from mutated or overexpressed disease- causing genes. [unreadable] [unreadable] [unreadable]